Inconsistent Illumination During Backlight-Only Test on Form 4

Support & Troubleshooting Form 4/4B
1

We’ve been facing a persistent and unresolved issue with our Form 4 printer related to inconsistent illumination during the backlight-only optical test.

What We’re Seeing:

  • Structured bright/dark circular zones appearing during the Backlight-Only test.
  • These zones are fixed in location, match the layout and size of the BLU LEDs, and are visible across multiple tests and days.
  • The issue persists even after thorough cleaning of the release texture (per Formlabs’ guidelines) and use of fresh A4 paper for the optical test.
  • The LPU is not active during this test, yet support is recommending replacing the LPU, not the Backlight Unit (BLU).

What We’ve Done:

  • Cleaned release texture thoroughly and documented it via video.
  • Repeated all diagnostics (optical tests, clean sheet tests).
  • Provided images and videos of test results.
  • Compared against Formlabs’ own video (https://www.youtube.com/shorts/L23V4o8OOno) which where they state the BLU provides “consistent and collimated illumination” — which our unit clearly does not replicate.

What We’re Asking:

  • Has anyone else experienced structured irregular lighting during the backlight-only test?
  • Can any Formlabs engineers or experienced users confirm whether this is normal behavior?
  • Why would there be non-uniform output from the BLU if it’s designed for uniformity?

We’ve reached out to Formlabs support, escalated the case, and requested to speak directly with an engineer. So far, support has been slow, and we’re losing valuable production time.

This was an incidental finding as a part of routine maintenance as we wanted to inculcate the machine into our main production workflow. We haven’t experienced any major print failures, having said that we suspect this could be an onset of failure of BLU.

Any input from the community — especially anyone who has run the backlight-only test under dark conditions — would be highly appreciated.

Thanks.

Backlight Only
Backlight Only1759×2476 249 KB

Clean Tank Trial 1
Clean Tank Trial 11405×2219 162 KB

Irregularly lit LEDs
Irregularly lit LEDs945×903 166 KB

Backlight Only Optical Test Comparision
Backlight Only Optical Test Comparision1436×974 243 KB

Clean Tank Trial 2
Clean Tank Trial 21236×1661 237 KB

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2

RESOLVED -

Formlabs Customer Support initially deduced the LPU to be faulty. We escalated the matter, as we needed technical reasoning and were not satisfied with the answers we received from customer support.

Eventually, we got through to the leadership team who reviewed the problem and accepted that there is variance in the backlight LED wavelength that affects uniformity. But assured that the BLU has passed their QC and will not affect the print quality.

They also did provide a technical explanation confirming that the faulty LPU was indeed the reason for our findings.

But surprisingly they decided to replace both the BLU and LPU.

After the replacement, we no longer see inconsistent illumination during the Backlight-Only test — the illumination is now even and consistent across the region with no bright spots. We are not certain which replacement solved the issue — whether it was the BLU, the LPU, or a combination of both.

Hope this helps :slight_smile:

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